Spark gaps have been used for many years in surge arresters to protect the insulation of utility high voltage power equipment from damage associated with system overvoltages, such as temporary power frequency and lightning surges. Conventional gapped arresters connected spark gaps in series with silicon carbide non-linear resistors. The spark gaps provided the spark over function while the silicon carbide resistors controlled the arrester follow current to a level that the spark gap could interrupt.
Other conventional arrester designs have utilized spark gaps with various combinations of spark gap grading components (connected directly in parallel with the spark gap) to control the voltage distribution across the spark gap(s) when exposed to various overvoltage waveforms. Proper control of the voltage distribution across the series gap(s) was needed to provide stable behavior of the arrester spark over characteristic.
Spark gaps have been designed with resistors, capacitors, or parallel combinations of resistors and capacitors, each component individually oriented in parallel with the spark gap. The primary purpose of the gap grading resistor was to control the voltage distribution between the series-connected gaps inside an arrester to assure that the arrester did not spark over under normal voltage power frequency conditions, including severely contaminated environments. Capacitors used in parallel with the spark gap(s) were instituted to cause a voltage upset between the arrester's series-connected gaps. This induced voltage upset, between spark gaps, controlled the spark over level of the arrester when exposed to voltage surges of varying frequencies (such as high frequency lightning and lower frequency switching surges) to a value that would protect equipment installations. Essentially, within the series gap structure, voltage was redistributed as a function of overvoltage and frequency of the surge.
More recently, arresters have been made with metal-oxide non-linear resistors (often referred to as metal oxide varistors, or MOV). MOV have improved non-linear resistor characteristics over the formerly used silicon carbide resistors, such that the spark gap used in silicon carbide arrester designs was eliminated.
A conventional gapless metal-oxide arrester contains for its voltage limiting feature a plurality of metal-oxide varistor (MOV) elements 1 stacked in series, as shown in FIG. 1. MOV elements are typically substantially cylindrical in shape and have a diameter typically in the range of approximately 25 mm to 75 mm and a height typically in the range of approximately 25 mm to 45 mm.
A distribution arrester with a duty cycle rated voltage of 9 kVrms and a maximum continuous operating voltage (MCOV) of 7.65 kVrms, for typical use on a 12.47 kVrms 3-phase distribution system, would typically contain two or three MOV elements 1. One particular example of such an arrester could contain two MOV elements, each having a diameter of 36 mm and a height of 35 mm, as shown in FIG. 1. Va is the total voltage impressed across the arrester. Vm is the fraction of Va that appears across each of the MOV elements 1.
Vm=0.5 Va for the case of two equal MOV elements, and this is virtually independent of the frequency of the applied voltage. Fast rising voltage impulses, such as those produced by lightning surges, have high frequency components that may be in the MHz range. For example, the time to crest of a voltage of frequency of 0.25 MHz is 1 microsecond, which is of the same order as the rise time of a fast rising lightning impulse voltage.
Another conventional distribution arrester includes an MOV element 2 electrically in series with a simple spark-gap 3, as shown in FIG. 2. Va is the total voltage impressed across the arrester. Vg is the fraction of Va that appears across the spark gap 3. Vm is the fraction of Va that appears across the MOV element 2. In the conventional arrester shown in FIG. 2, with no grading components connected across the spark gap 3, virtually all the voltage Va appears across the gap under normal operating conditions; i.e. Vg=Va and Vm=0, where Va=Vg+Vm.
At a sufficiently high value of Va (for example, during an overvoltage surge caused by a lightning stroke on or near the distribution system), the spark gap 3 will spark over and all the voltage will be impressed on the MOV element 2. The MOV element 2 conducts the surge current and limits the impressed voltage according to the non-linear volt-amp characteristic of the MOV element 2. This type of gapped arrester has the advantage that under normal system operating conditions, there is no voltage across the MOV, and therefore there are no power losses (this is in contrast to the gapless design of FIG. 1). A further advantage is that this type of gapped arrester can use a considerably reduced amount of MOV elements 2 compared to a gapless arrester (FIG. 1) because there is no power frequency voltage across the MOV under normal operating conditions. When there is a high voltage surge that causes the spark gap 3 to spark over, all the voltage is transferred to the MOV element 2. The duration of the surge is sufficiently short such that the MOV element 2 can survive the imposed electrical stresses, and after the surge ends the gap “re-seals” to again remove the power system voltage from the MOV element 2. An advantage of using a reduced amount of MOV elements 2 compared to a gapless arrester (FIG. 1) is that the surge voltage limitation is improved; i.e. the surge voltage is limited to a lower level for the same amount of surge current. For example, if the gapped arrester uses only one MOV element compared to two MOV elements of the gapless arrester, the limiting voltage of the gapped arrester (after the gap sparks over) will be 50% of the limiting voltage of the gapless arrester for the same surge current. This is advantageous in that the equipment being protected by the arrester will experience reduced levels of surge voltage for the case of the gapped arrester, providing an improved “margin of protection”.
However, a significant disadvantage of the gapped arrester of FIG. 2 is that it is difficult to control the spark over behavior of the spark gap 3. Simple gaps possess a characteristic that the voltage required to spark over the gap increases with frequency. Fast rising voltage impulses, such as those produced by lightning surges have high frequency components that may be in the MHz range. For example, the time to crest of a voltage of frequency of 0.25 MHz is 1 μs, which is of the same order as the rise time of a fast rising lightning impulse voltage. The voltage required to spark over the gap for a fast rising lightning impulse can be greater than the voltage to which the MOV element will limit the voltage after the gap sparks over. The equipment being protected will experience this higher voltage, and the “margin of protection” is reduced accordingly. In certain cases, for very fast rising surges, the gap spark over voltage can exceed the limiting voltage of a gapless arrester (FIG. 1), in which case the gapped arrester (FIG. 2) performs an inferior job of protection compared to the gapless arrester.
To partially overcome the problem with spark over characteristics of a simple spark gap, another conventional arrester includes a resistive grading element 4, Rg, connected across the spark gap 5, as shown in FIG. 3. Va is the total voltage impressed across the arrester. Vg is the fraction of Va that appears across the spark gap 5. Vm is the fraction of Va that appears across the MOV element 6. Rg is typically in the range of 40-100 MΩ While this helps address the gap spark over control issue, it re-introduces the issue of power losses. Power losses of this type of design can easily be greater than those of gapless designs of the same MCOV.